Well, the oven was off (in fact, it hadn't been used for many hours)
and all of a sudden it starts emitting a high pitched whistle. We were
in the dining room, and thought it was a smoke alarm in the house
until we identified it as coming from the microwave. The digital
display was completely bonkers with nonalphameric characters. The only
way we could shut it off was to pull the plug on it. None of the
buttons worked.

This happened several times and finally (after power cycling) we were
able to get it to accept a new clock time, and behave itself. Never
had any problems with this over over a decade.

Umm, is this a known failure mode? I'd figure that if the
microprocessor goes south, the result would be less
spectacular.

Whoops, there it goes again.

I gave the console a whack, and it stopped. Must be a loose card?